A switching regulator (or converter) is a circuit that uses a power switch, an inductor, and a diode to transfer energy from input to output. At the output, a storage capacitor receives the energy generated in the inductor via switching of the power switch. The basic components of the switching converter can be rearranged to form a step-down (buck) converter, a step-up (boost) converter, or an inverter (flyback), for example. Feedback and control circuitry can be nested around these circuits to regulate the energy transfer from the inductor to the storage capacitor to maintain a constant output within normal operating conditions.
The most common control method for controlling the switching converter is via pulse-width modulation (PWM). This method takes a sample of the output voltage and subtracts this from a reference voltage to establish a small error signal (VERROR). This error signal is compared to an oscillator ramp signal. A comparator outputs a digital output (PWM) signal that operates the power switch. When the circuit output voltage changes, VERROR also changes and thus causes the comparator threshold to change. Consequently, the output pulse width (PWM) also changes. This duty cycle change then moves the output voltage to reduce the error signal to zero, thus completing the control loop.
One issue with PWM converters is related to operating stability of the control loops that maintain the output voltage in regulation. The control loop for the converter has a gain factor that varies over the operating frequency of the converter. The gain factor is influenced by Poles and Zero's that are derived from the transfer function of the circuit and are influenced by the different components of the converter. For example, the output storage capacitor in the converter has a parameter referred to as equivalent series resistance (ESR) which is derived from the frequency in which the storage capacitor receives energy from the input side of the converter. The ESR of the storage capacitor contributes a Zero in the control transfer function which contributes to the overall stability of the control loop (e.g., cancels a dominant Pole in the loop). Over time and temperature, however, the ESR of the storage capacitor can change which changes the placement of the Zero in the frequency domain for the control transfer function and thus can destabilize the control loop.